1. Field of the Invention
The present invention pertains to a lightning protective device for mobile radar antenna, a device that does not disturb the functioning of the antenna by its presence in the said antenna's transmission or reception sector.
2. Description of the Prior Art
Radar antennas have parts that are very vulnerable to lightning: these parts are the radiating elements and the electronic components that are located behind these radiating elements.
Antennas with one or more reflectors are far less fragile than electronic scanning antennas. For a reflector is a metallic part, generally a grid, that is insensitive to lightning, conducts electricity and is electrically connected to the ground: it gives partial lightning protection to the radiating elements and electronic components. In other words, it acts as a lightning conductor. No part of an electronic scanning antenna can fulfil this type of role.
Electronic scanning antennas which are, moreover, very expensive, must imperatively be protected from lightning.
An object is normally protected from lightning by a lightning conductor consisting of one or more fixed metallic rods which are located near the object to be protected, overhang the said object and are electrically connected to the ground, thus making it possible to remove heavy electric charges.
Unfortunately, a device of this type is ill-suited to the lightning protection of a radar antenna. For a radar antenna transmits or receives energy in several sectors of space. In particular, certain antennas explore every direction in the horizontal plane. Now an ordinary lightning conductor has at least one fixed metallic rod which overhangs the antenna to be protected and which is electrically connected to the ground by at least an electrically conductive element which is, itself, also generally metallic. The set comprising this element and the rod are therefore inevitably, at a given moment, in the transmission or reception sector of the antenna, and its presence in this sector greatly disturbs the functioning of the antenna.
In the prior art, there are collapsible lightning conductors which are put away in fine weather and which are set up when there is a danger of storms. The erection of the lightning conductor is controlled by a storm detector which measures the electrical field and the magnetic field of the surrounding atmosphere. An antenna protected by a lightning conductor of this type works normally in fine weather and can only be disturbed when there is a threat of stormy weather which is taken to be brief in comparison with fine weather.
However, a collapsible lightning conductor has disadvantages. First, it is a costly device. Second, it requires the use of storm detectors which add to the cost. Secondly, it is a device that does not resolve the problems raised by a lightning conductor which is fixed in a highly keronic region, i.e. a region where there is a high frequency of storms as, for example, in a tropical zone.